Monika- It could just be that he is still recovering from being sick. Sometimes when people (PKU and non PKU) get sick their bodies can go into catabolism. This is where their bodies break down protein from the bodies muscles to get energy. Do you know if he's getting enough calories? If he's going through a growth spurt or if he is more active then usual that could also cause a difference in his phe levels. Have you talked to Jessica? If not it might be a good idea to email her and see what she has to say.
I honestly cannot tell you how high my phe levels get when I am sick. I usually avoid doing levels when I'm not feeling well, because I already know they will be higher than normal.